As shown in FIG. 8, an antenna device includes an antenna unit 43, a directional coupler 42, a receiving unit 40, a transmitting unit comprising power amplifiers 32 and 37 and attenuators (ATTs) 38 and 39, and a switch 41 for switching transmit/receive. The attenuators (ATTs) 38 and 39 are connected to the inputs of the power amplifiers 32 and 37, respectively. An input power passes through the attenuator (ATT) 39 and is amplified by the power amplifier 37 and, then, the amplified input power passes through the attenuator (ATT) 38 and is amplified by the power amplifier 32, to be transmitted to the antenna unit 43 as an output power.
As shown in FIG. 9, the power amplifier 32 includes an input matching circuit 9 connected to an input terminal 11, an output matching circuit 10 connected to an output terminal 12, and a transistor 33 connected between these matching circuits. The gate of the transistor 33 is connected to a gate bias terminal 35, and the drain thereof is connected to a drain bias terminal 36. Further, coils 29 and 31 for cutting off RF components are connected to the gate bias terminal 35 and the drain bias terminal 36, respectively.
Generally, in order that an antenna device realizes accurate detection of a specified object, it is necessary to make an output power of a transmitting unit constant. Especially in an array antenna device in which a plurality of antenna devices shown in FIG. 8 are arranged, when the transmission levels of the respective antenna devices are not constant, the whole transmission level of the array antenna device is unstable.
As to the power characteristics of the transistor in the power amplifier 32, as shown by a curve 16 in FIG. 7, when input power P.sub.in is lower, output power P.sub.out increases proportionally to the input power P.sub.in, and when the input power P.sub.in becomes higher, the output power P.sub.out attains maximum value at an input power value. Accordingly, the power amplifier 32 has such power characteristics as well. Consequently, the output power P.sub.out varies with variations in the input power P.sub.in according to frequency characteristics and the temperature. For this reason, in the conventional device, as shown in FIG. 8, the attenuator (ATT) 38 is connected to the input of the power amplifier 32 to adjust the level of the input power P.sub.in. However, in the device in which the attenuator (ATT) 38 adjusts the input power P.sub.in, it is difficult to make the output powers constant with respect to a wide range of the extensive input power levels.
In addition, in order to make the sensitivity of the antenna device better, the antenna device is usually used at the maximum output level of the transmitting unit. Therefore, the input power P.sub.in of the power amplifier 32 is set to a value at which the output power P.sub.out is maximum. The power added efficiency of the power amplifier 32 is obtained from a formula: (output power P.sub.out --input power P.sub.in)/power of drain bias. This power added efficiency decreases, as shown by a curve 19 in FIG. 7, rapidly when the input power P.sub.in is excessive input, i.e., when the input power P.sub.in exceeds a region where the output power P.sub.out is maximum. Consequently, the power added efficiency significantly decreases when the input power P.sub.in is excessive input, so that the consumption power of the power amplifier increases.